Ruby Modules


Modules are a way of grouping together methods, classes, and constants. Modules give you two major benefits.

Modules define a namespace, a sandbox in which your methods and constants can play without having to worry about being stepped on by other methods and constants.

Syntax:
      module Identifier
         statement1
         statement2
         ...........
      end
      

Module constants are named just like class constants, with an initial uppercase letter. The method definitions look similar, too: module methods are defined just like class methods.

As with class methods, you call a module method by preceding its name with the module.s name and a period, and you reference a constant using the module name and two colons.

Example:
      #!/usr/bin/ruby
      
      # Module defined in trig.rb file
      
      module Trig
         PI = 3.141592654
         def Trig.sin(x)
         # ..
         end
         def Trig.cos(x)
         # ..
         end
      end
      

We can define one more module with same function name but different functionality:

      #!/usr/bin/ruby
      
      # Module defined in moral.rb file
      
      module Moral
         VERY_BAD = 0
         BAD = 1
         def Moral.sin(badness)
         # ...
         end
      end
      

Like class methods, whenever you define a method in a module, you specify the module name followed by a dot and then the method name.

 

Ruby require Statement:

The require statement is similar to the include statement of C and C++ and the import statement of Java. If a third program wants to use any defined module, it can simply load the module files using the Ruby require statement:

Syntax:
      require filename
      

Here it is not required to give .rb extension along with a file name.

Example:
      require 'trig.rb'
      require 'moral'
      
      y = Trig.sin(Trig::PI/4)
      wrongdoing = Moral.sin(Moral::VERY_BAD)
      

IMPORTANT: Here both the files contain same function name. So this will result in code ambiguity while including in calling program but modules avoid this code ambiguity and we are able to call appropriate function using module name.

 

Ruby include Statement:

You can embed a module in a class. To embed a module in a class, you use the include statement in the class:

Syntax:
      include modulename
      

If a module is defined in separate file then it is required to include that file using require statement before embeding module in a class.

Example:

Consider following module written in Week.rb file.

      module Week
         FIRST_DAY = "Sunday"
         def Week.weeks_in_month
            puts "You have four weeks in a month"
         end
         def Week.weeks_in_year
            puts "You have 52 weeks in a year"
         end
      end
      

Now you can include this module in a class as follows:

      #!/usr/bin/ruby
      require "Week"
      
      class Decade
      include Week
         no_of_yrs=10
         def no_of_months
            puts Week::FIRST_DAY
            number=10*12
            puts number
         end
      end
      d1=Decade.new
      puts Week::FIRST_DAY
      Week.weeks_in_month
      Week.weeks_in_year
      d1.no_of_months
      

This will produce following result:

      Sunday
      You have four weeks in a month
      You have 52 weeks in a year
      Sunday
      120
      

 

Mixins in Ruby:

Before going through this section, I assume you have knowledge of Object Oriented Concepts.

When a class can inherit features from more than one parent class, the class is supposed to show multiple inheritance.

Ruby does not suppoprt mutiple inheritance directly but Ruby Modules have another, wonderful use. At a stroke, they pretty much eliminate the need for multiple inheritance, providing a facility called a mixin.

Mixins give you a wonderfully controlled way of adding functionality to classes. However, their true power comes out when the code in the mixin starts to interact with code in the class that uses it.

Let us examine the following sample code to gain an understand of mixin:

      module A
         def a1
         end
         def a2
         end
      end
      module B
         def b1
         end
         def b2
         end
      end
      
      class Sample
      include A
      include B
         def s1
         end
      end
      
      samp=Sample.new
      samp.a1
      samp.a2
      samp.b1
      samp.b2
      samp.s1
      

Module A consists of the methods a1 and a2. Module B consists of the methods b1 and b2. The class Sample includes both modules A and B. The class Sample can access all four methods, namely, a1, a2, b1, and b2. Therefore, you can see that the class Sample inherits from both the modules. Thus you can say the class Sample shows multiple inheritance or a mixin.